1. Field of the Invention.
This invention relates to a hydraulic brake actuator for a wedge type brake and, more specifically, to such a brake actuator which utilizes an actuation rod and an improved sealing configuraiton to prevent the escape of hydraulic fluid around the actuation rod.
2. Description of the Prior Art.
There has heretofore been provided a number of emergency and service hydraulic brake actuators such as those disclosed in U.S. Pat. Nos. 3,232,175, 3,292,423, 3,462,986; and No. Reissue 26,432. These actuators all include an emergency brake actuation member which is mounted on a tubular structure which extends from the brake and houses an extended end of the stem of the wedge of the brake therein. A primary cylinder of the emergency actuation member is mounted on the tubular structure and includes a primary piston mounted for sealed movement within the primary cylinder. The primary piston includes a rod which extends from the first side of the piston into the tubular structure to be operably connected to the extended end of the stem. A spring acts on the second side of the primary piston and tends to produce emergency actuation of the brake if it is not opposed by primary hydraulic fluid introduced to the primary cylinder at the first side of the primary piston.
In these combined emergency and service hydraulic brake actuators, the operable connection between the emergency actuation member and the wedge of the brake included a means for introducing service hydraulic fluid into the interior of the tubular structure. The service hydraulic fluid would act on the end of the rod and on the end of a stem of the wedge to selectively increase the relative distance therebetween during service actuation of the brake. This configuration required that the tubular structure be designed to withstand service hydraulic fluid pressure and that there be sealing provided to prevent the escape of the service hydraulic fluid into the emergency actuation member or the brake. Specifically, in order to separate the interior of the primary cylinder and the interior of the tubular structure, the rod from the primary piston was required to extend through a common wall therebetween and to be sealed to prevent the passage of hydraulic fluid from either interior into the other. In these prior art devices, the sealing around the rod was provided by one or more O-rings. However, it has been found that the O-ring seals in these and similar actuator configurations have not always had the desired life expectancy needed for safe and reliable brake operation. Whether the decreased effectiveness of such seals is due to an inability to maintain proper tolerances for such an installation, the wear of such O-rings due to repeated axial movement of the primary piston rod, or other factors affecting the wear of the O-rings such as the requirement that opposite sides of the O-rings are subjected to different hydraulic fluid pressure levels is not fully known and might vary from configuration to configuration.
It should be noted that a similar sealing problem could exist in certain emergency actuators which are not used in conjunction with the type of service actuation means as found in the above-mentioned patents. A spring-applied, hydraulic-released actuator could be employed in a wedge type brake for emergency braking without there being provided a service actuator therebetween for service braking. Service braking could be separately provided within the brake without employing the wedge itself. However, there would still be a need to insure that there is adequate sealing around the actuation rod to prevent primary hydraulic fluid from escaping into the tubular structure even though the tubular structure would not be designed to withstand or include any service hydraulic fluid.
One prior art emergency actuator without a combined service feature has also used O-ring seals around the actuation rod. However, the primary piston rod does not extend through a wall at the end of the primary cylinder which was integrally formed with the primary cylinder as was the common wall in each of the actuators disclosed in the patents mentioned above. Instead, an annular seal carrier is mounted around the rod to prevent the escape of hydraulic fluid from the primary cylinder through an annular space between an interior surface of the tubular structure and the exterior surface of the rod. Specifically, the annular seal carrier has a circumferential groove and O-ring configuration about its exterior surface to seal against the interior surface of the tubular structure and a circumferential groove and O-ring configuration about its interior surface to seal against the exterior surface of the actuation rod.
It is expected that such an O-ring configuration could have the same life expectancy problems as the O-ring configurations shown for the integral common wall of the actuator in the patents mentioned above. However, the use of a separate sealing carrier might simplify assembly and be otherwise attractive since the grooves would be easier to form and the O-rings easier to install on a carrier than they could within the integral wall of the larger primary cylinder. On the other hand, the use of a separate sealing carrier does increase the number of components which require finer manufacturing tolerances to insure an effective seal. With the integral wall and only one O-ring, the tolerances of the interior surface of the wall opening, the base of the groove, and the exterior surface of the rod are critical to insure adequate sealing. With the separate annular seal carrier with one O-ring at its exterior surface and one O-ring at its interior surface, the tolerances of the interior surface of the tubular structure, the exterior surface of the carrier, the base of the groove at the exterior surface, the interior surface of the carrier, the base of the groove at the interior surface, and the exterior surface of the rod are all critical. Of course, if an additional O-ring is desired between any of the interior and exterior surfaces, the base of the particular groove needed for the additional O-ring must also be provided the desired tolerances.
Each component requiring finer tolerances adds to the overall cost of providing an effective actuator and any failure to provide the necessary tolerances for any one component reduces the likelihood of effective sealing for the actuator.